Heat resistant black fibers and fabrics derived from regenerated cellulose,containing certain heavy metals

ABSTRACT

Black, essentially amorphous, electrically conducting, carbonaceous fiber materials containing about 0.5 to 6 percent by weight of nitrogen, from 1 to 50 percent by weight of a heavy metal and about 40 to 95 percent by weight of carbon, the said heavy metal being present in a form other than the carbide, and being chemically bound to the said fiber, are obtained by the pyrolysis of regenerated cellulose fibers which are impregnated with certain salts under conditions in which exotherm is prevented and below the temperature at which any substantial reaction of the heavy metal with carbon occurs to form refractory carbide. An intermediate non-conducting carbonaceous fiber material can also be obtained.

United States Patent Hart [ Apr. 9, 1974 [75] Inventor: Marvin L. Hart,Stillwater Township, Washington County, Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul,Minn.

[22] Filed: Jan. 27, 1972 [2]] Appl. No.: 221,832

Related 0.8. Application Data [62] Division of Ser. No. 740,987, June28, I968,

abandoned.

[52] US. Cl. 252/478, 117/46 CC, 252/30l.l R, 264/.5, 264/29, 423/439,423/440 [51] Int. Cl C09k 3/00 [58] Field of Search 252/478, 301.1 R;264/.5,

264/29; 423/439, 440; Il7/46 CC [56] References Cited UNITED STATESPATENTS 3.242.000 3/1966 Lynch 264/.5 UX

HEAT RESISTANT BLACK FIBERS AND FABRICS DERIVED FROM REGENERATEDCELLULOSE, CONTAINING CERTAIN I-IEAVY METALS 3,403,008 9/ 1 968 Hamling264/.5

Primary ExaminerLeland A. Sebastian fittorney, Agent, or FirmAlexander,Sell, Steldt & DeLaHunt I I I [57] ABSTRACT Black, essentiallyamorphous, electrically conducting. carbonaceous fiber materialscontaining about 0.5 to 6 percent by weight of nitrogen, from 1 to 50percent by weight of a heavy metal and about 40 to 95 percent by weightof carbon, the said heavy metal being present in a form other than thecarbide, and being chemically bound to the said fiber, are obtained bythe pyrolysis of regenerated cellulose fibers which are impregnated withcertain salts under conditions in which exotherm is prevented and belowthe temperature at which any substantial reaction of the heavy metalwith carbon occurs to form refractory carbide. An intermediatenon-conducting carbonaceous fiber material can'also be obtained.

22 Claims, No Drawings HEAT RESISTANT BLACK FIBERS AND FABRICS DERIVEDFROM REGENERATED CELLULOSE, CONTAINING CERTAIN HEAVY METALS This is adivision of application Ser. No. 740,987, filed June 28, 1968 nowabandoned.

BACKGROUND OF THE INVENTION Fibers of carbon or graphite are known, andhave been produced by carbonization of a number of materials, with orwithout subsequent heat treatments to bring about crystallization of thecarbon. Likewise, fibers which are essentially metal carbides have beenprepared, as have metallic nitride fibers and the like. Such fibers,however, are not the same as nor do they have the same properties asfibers of the instant invention. Furthermore, the presently claimedprocesses for making fibers containing heavy metals are likewise novel.Typical disclosures of the fibers of the prior art can be found inpatents to Peters, U.S. Pat. No. 3,235,323; Cross et al., U.S. Pat. Nos.3,116,975; 3,281,261; British Pat. specification No. 1,012,878 and thelike. The Peters U.S. Pat. No. 3,235,323 shows a process similar to thatof the present invention, but the inclusion of heavy metals in thatprocess is not suggested. It might have been expected that inclusion ofheavy metals would catalyze combustion so as to render pyrolysisuncontrollable. but surprisingly it has been found that this is not thecase.

Carbonized acrylic or rayon textile products including refractory metaloxides are disclosed in U.S. Pat. No. 3,242,000. Such materials, whilethey contain refractory metals as oxides, are different from the fibersproduced in the process of the present invention in that they do notcontain combined nitrogen and in that they apparently contain therefractory metals as oxide coatings. These materials, as well as thosemade by longcontinued heating of carbonizable fabrics, are not the sameas or even closely similar to applicants amorphous materials, made byextremely rapid heating with short dwell time, in which the heavy metalsare somehow bound into a residual, amorphous polymer chain which iscomposed chiefly of carbon with a small percentage of combined nitrogen.

The present invention contemplates providing carbonized fibers producedby pyrolysis of regenerated cellulose fibers, containing by weight fromabout /z to 6 percent of nitrogen and from 1 to about 50 percent ofheavy metal combined with the said carbonized fiber, the amount ofcarbon contained in said fiber being from about 40 to about 95 percent(depending upon the amount of heavy metal present). Possibly very minoramounts of other constituents (totaling not more than percent) can alsobe present in the carbonized fibers. Preferably these final carbonizedfibers of the present invention contain /2 to 4 percent of nitrogen andl to 35 percent of heavy metal with the remainder (except for very minoramounts of other constituents) being carbon. Also within the purview ofthe invention are processes for producing such fibers containing heavymetals.

The novel fibrous materials of the invention are produced by thethermochemical transformation of corresponding regenerated cellulose,e.g. rayon, fabrics and fibers, which retain their fibrous identity butare changed to a flexible black state having an entirely differentchemical composition and radically different chemical and physicalproperties. In the process, a regenerated cellulose precursor materialis impregnated with a nitrogenous salt and with a heavy metal salt or acombination of salts of' two or more heavy metals, dried to removesubstantially all of the solvent (e. g. water), and then heated in aprocess comprising either one or two stages of pyrolysis. In the twostage pyrolysis, the first stage is carried out in the presence of airat about 200-300C. and the second stage is carried out in an inertatmosphere at a final temperature of at least about 700 C., preferablyin the range of 1,200l,500 C. In the one stage pyrolysis, the lowertemperature step is eliminated. Thus the impregnated precursor materialis heated rapidly in a non-oxidizing environment at a temperature of atleast 700 C., preferably in the range of 1,200-l,500 C.

The final product of the processes is a black, carbonaceous materialwhich is electrically conductive, amorphous, highly heat resistant andwhich has significant capability for absorption of x-radiation.

In the first stage of heating in the two step pyrolysis process (whichis presently preferred as giving a stronger final product), theimpregnated fibers are not only changed by pyrolysis but by anaccompanying chemical reaction wherein nitrogen atoms become chemicallycombined in some way in the stable polymeric oxygen-containing carboncompound molecules of the black fibers produced. The thermochemicaltransformation is rapid, requiring not more than about 1 to 10 minutesat temperatures in the range of about 200-300C., and is accomplished inthe presence of oxygen under conditions controlled to prevent rapid anddestructive exotherm from taking place. These include control of theflow of the atmosphere as well as close control of final temperature.Under the preferred conditions, a dwell time of as little as 2 minutesor less is employed. The regenerated cellulose fibers are rapidlybrought to the oven temperature as they are conveyed directly into thezone heated to pyrolysis temperature. The process is therefore quitedifferent from those processes previously employed in producingpartially carbonized fibers and carbon fibers, by lengthy heating ofcellulose fibers at gradually increased temperatures.

During this stage of the chemical transformation, water, carbon dioxide,carbon monoxide and other materials, such as decomposition products ofnitrogen salts which may be present, are evolved. The result is anintermediate fibrous product which contains (in percent by weightexclusive of the heavy metal) at least A percent and preferably not morethan 15 percent of nitrogen, 50 to 65 percent of carbon (from thefiber), the remainder including hydrogen, oxygen and other residuals,e.g. from the impregnating salts. The total amount of heavy metal in theintermediate product is essentially the same as in the originalimpregnated fiber, none having been lost in the first pyrolysis step.

While as noted hereinabove, dwell times of from 1 to 10 minutes attemperaturesin the range of about 200-350 C. are employed in the firststage of the process, higher temperatures and shorter dwell times may beemployed. Thus, for example, if the temperature is raised to the orderof 400500 C. or even somewhat higher, a dwell time which is considerablyshorter, even of the order of seconds, can be employed. It will ofcourse be apparent that such short dwell times will be most useful inthe case of conversion of fibers having small mass, i.e. single ply ortwo ply yarns of small denier, in that the pyrolytic conversion occursthroughout the cross-section of the fibers during the short dwell time.In such instances, however, it may be necessary to adjust the ambientatmosphere to prevent uncontrolled combustion. This can be done byrestricting the flow of air, whereupon the oxygen content of theatmosphere surrounding the fibers is lowered.

ln the second stage of pyrolysis in the two step process, theintermediate product is heated in a nonoxidizing atmosphere, preferablyargon, other rare gases or the like, although nitrogen, or mixtures ofnitrogen and argon, etc. can be employed. The essential consideration isto permit evolution of gases from the fibers during continued pyrolysis,without reaction with the ambient atmosphere. The temperature of thefiber is rapidly brought to the range of about l,200-l,500 C., and evensomewhat higher, but below the temperature at which any substantialamount of reaction of the heavy metal employed with carbon would takeplace to produce heavy metal carbide. Heating of the fiber in this stageis also rapid, from 5 seconds to minutes being required, and dwellperiods of 10 to 60 seconds being the preferred condition.

In the processes of the present invention, it appears that theregenerated cellulose is first degraded, apparently by decomposition andring-cleavage of the cellulose molecules; the result being liberation ofwater vapor and other gases. As the reaction progresses, the decomposedmolecules recombine to form a new type of polymeric molecule, containingcarbon-bonded nitrogen atoms and also apparently containing carbon ornitrogen-bonded heavy metal to the extent that this was present in theoriginally impregnated fiber.

As noted previously, the lower temperature stage in the pyrolysis can besimply eliminated and the higher temperature stage carried out in themanner explained above.

The water-soluble nitrogenous salt which is employed to impregnate theregenerated cellulose fibers and/or fabric is a non-oxidant,water-soluble salt of a strong acid and nitrogenous base, e.g., ammoniumchloride, methyland ethyl-amine salts of phosphoric acids, and mixturesthereof. To this may be added boric acid or an equivalent boroncompound, if desired. The salts useful for this process of the inventionare those disclosed in the US. Pat. No. 3,235,323, for making carbonizedrayon fibers and fabrics which do not contain heavy metals.

The material to be pyrolyzed is impregnated with about 5 to about 45percent of the salt, on a dry weight basis for the best results. Thismay be accomplished by immersion for about 5 minutes or less in a nearlyboiling salt solution, the excess solution then being removed. The wetpickup of salt solution is about 200 percent by weight.

The heavy metal salts employed for impregnation are salts ofcarbide-forming heavy metals, which have carbides stable at hightemperatures. They are to be distinguished from the metals which rejectcarbon, for example, copper, silver, tin, germanium, lead and so on.Reference is made to Kirk-Othmer Encyclopedia of Chemical Technology,Vol. 4, 1964, pages 75-91, also to page 71 of that publication, whereinthe binary compounds of carbon are classified. Included within the heavymetals therein shown and categorized with respect to carbide formationare tungsten, titanium, tantalum, niobium, chromium, molybdenum,vanadium, hafnium, zirconium, uranium and thorium. Fissionable actinidescan also be used. The soluble salts of these metals useful for thepurpose of the invention include salts with strong and weak acids,preferably those which have high solubility in water.

Where the solubility of the heavy metal salt is great enough, this canbe included in the solution of salt of strong acid and nitrogenous basepreviously referred to. However, particularly where larger amounts ofheavy metal are to be included in the fiber, it is necessary tointroduce the heavy metal salt in a separate step (drying the fiberbetween the impregnations). Introduction of the heavy metal saltpreferably precedes but may follow the impregnation with nitrogenoussalt. Again dependent upon whether larger percentages of salt are to beintroduced into the regenerated cellulose, the solutions can be warmedor even heated to boiling, to facilitate impregnation of the fibers.

The amount of salt to be added to the fiber on a wet or dry basis is ofcourse dependent upon the desired concentration of heavy metal in thefinal product. As a guideline, 5 percent by weight of heavy metalintroduced into the initial fiber (on a dry weight basis) normallyresults in a concentration of about 20 percent of heavy metal in thefinal product.

It is considered to be quite unexpected that the presence of the heavymetal does not interfere with the chemical transformation of theregenerated cellulose in the presence of nitrogenous salts.

Typical heavy metal salts which can be employed are zirconiumtetracetate, tungstic oxide (in ammoniacal solution), tungstosilicicacid, uranium nitrate, thorium nitrate, molybdenum oxalate, tantalumfluoride, titanium tetrachloride, titanium oxalate, tungstentetrachloride, chromium acetate, molybdenum chloride, hafniumtetrachloride, and the like. Mixtures of these salts can be used so thatthe ultimate carbonaceous material contains two or more heavy metals.

The damp, salt-treated fibrous material is then heated until dry,preferably at about to 200 C. It is found that the dry fibers areimpregnated with the salts used, not merely coated. If the impregnationtreatment has caused the fibers to stick together, processing isimproved by flexing, rubbing, tumbling and the like to loosen and freethe individual fibers so they will be fully exposed to the ambientatmosphere during heatmg.

The dried, salt-impregnated regenerated cellulose fibers are then heatedin a suitable oven to bring about the transformation of the fibers.

EXAMPLE 1 The following procedure, in which all parts are by weightunless otherwise specified, illustrates a presently preferred procedurefor making carbonized regenerated cellulose fiber or cloth containingheavy metals. The material used is first cleaned by the usual methods,e.g., by scouring, washing, detergent treatment or the like, to removesizings, lubricants, etc. This step may be omitted if the material isclean as received from the supplier.

Two-ply rayon yarn (regenerated cellulose, denier 1,650 per end, 720filaments per end) is thoroughly impregnated by passing it through a diptank containing a solution including ammonium chloride or sulfate and asoluble salt of the metal to be introduced into the fibers. A usefulsolution of this type is the following:

Ammonium chloride parts Zirconium tetraacetate 10 parts Acetic acid(glacial) 3 parts Wetting agent 0.3 parts Water 66.7 parts The wettingagent used is not critical. It may be omitted if desired; any compatiblewetting agent can be used, in small amounts as known to the art, tofacilitate wetting of the fibers and penetration of the solution.

Conveniently, the yarn is handled in long lengths which are stored onrolls. The yarn is usually passed through tube-like ovens, electricallyheated, having suitable control means. These are located so that theprocess steps are performed sequentially, the length of the heated zonesbeing such as to provide the requisite dwell time with the yarntravelling at constant speed of, e.g., l to 2 feet per minute.

The impregnated yarn is passed through a drying oven which is suppliedwith heated air at 140 C. flowing at a rate of 30 CPI-I, the temperatureof the drying tube being maintained at about 155 C. After substantiallyall of the water has been removed from the yarn, which is accomplishedby a dwell time of about one minute, it is conveyed into and through anoven which is supplied with heated air at about 200 C. which is flowingat the rate of about 30 CFI-I. The temperature in this oven ismaintained at about 245 C. The yarn is exposed to the oven temperaturefor 2 minutes. The yarn, transformed by the heating to a black, lustrouscondition, emerges from the oven into ambient air. At this point, thematerial is electrically non-conductive and quite strong. The weightpercent of heavy metal originally present has increased about 50percent, and the yarn itself has decreased to about 80 percent of itsoriginal weight. However, owing to the presence of the heavy metal,which is strongly incorporated into the rayon, so that it can no longerbe washed out, the material exhibits excellent absorption of x-ray orgamma radiation. The yarn is also heat resistant and can be heated to400 C. for short periods of time in air without loss of integrity.

The yarn thus obtained may be used as such for various purposes, beingheat resistant and x-ray absorbent, but it is preferred to treat thematerial by further pyrolysis, to obtain a material having a greatlyincreased proportion of carbon and which is electrically conductive. Forthis purpose, the yarn is passed into and through an oven which isprovided with an argon atmosphere, flowing at the rate of about 12 CFH(cubic feet per hour), and which is maintained at l,370 C. The yarn isexposed to the oven temperature for 40 seconds. 1mmediately afteremerging from the oven, the yarn is exposed to ambient temperature andatmosphere.

The yarn thus produced (impregnated with the above solution) contains12.7 percent zirconium, 84.6 percent carbon and 1.9 percent nitrogen.About 0.6 percent of hydrogen is also present. The material iscompletely amorphous and is found to have a breakingstrength of 3,405grams, denier 1,755 and tenacity 1.98. The conductivity of this materialis about ohms/ft. The yarn is very heat-resistant, and strongly absorbsx-radiation.

A similar procedure is used with other heavy metals which it is desiredto incorporate into the rayon yarn. In the event that the heavy metal isnot soluble to the extent desired in the solution containing theammonium compound, it is preferred that the heavy metal be firstdeposited in the rayon fibers by way of a solution, preferably aqueousbut which may also contain organic solvents, followed by drying and thenimpregnating with the ammonium compound-containing solution. In thisway, heavy metals, salts of which are relatively less soluble in thesolution of ammonium compound, can be incorporated in larger amounts.

The atmosphere in the first heatingstage, where the rayon yarn is heatedto approximately 250 C., is carefully controlled so as to preventexotherm, glowing and uncontrolled combustion of the rayon yarn. Thiscan be done visually if desired, or by using sensitive thermometricdevices. The atmosphere in this stage consists of decomposition productsof the rayon yarn together with air. If desired, ammonia or inert gascan be introduced to reduce or prevent exotherm, or the flow of freshgas into the oven can be reduced'to permit the percentage of atmospherecontributed by the off-gases to be increased. It will be recognized thatsome of the heavy metals are more likely to catalyze exotherm anddecomposition than others, and appropriate precautions can be taken,following simple experimental runs to determine empirical values foratmosphere, temperature and flow. The inclusion of boric acid orequivalent boron compound in the impregnating composition to provide 0.5to 2.5 percent boron content in the yarn also inhibits combustion andglowing.

The following Table I shows the results obtained when rayon yarnsimpregnated with various heavy metals of the kind used in making thecarbonized materials according to the invention were treated by theabove First heating Second heating Drying section stage stageImpregnating solution Atrn./ Atn1./ Atm/ Analysis, percent Break Te-Heavy metal Temp., flow, Temp., flow, Temp, flow, stren th I )enac- Runcompound/parts Other additive 0. 0.1.11. C. c.f.h. "C. c.l'.h. C N Metalg.) mer ity 1 Uranyl nitrate/10 Air/50... 242 Air/30." 1,426 Nz/S 84.41.4 12.1 1,635 1,590 1.03

2 'Ietrabutyl (Ethanol 165 Air/50.-. 220 Air/50.-. 1,426 Ni/8 11.1 3,3601,404 2.39

titanate/30. solution).

3 Nizrabiumoxalatel (Oxa'c acid/5L... Air/50. 210 N2/30 1,426 Nz/B 77.31.3 14.6 545 1,623 0.34 4 Hafnium tetra- (Ethanol Air/50. 93 Air/30-..1,426 Ni/B 78.0 1.6 19.7 1,635 1,819 0.90 chloride/10. solution).

5 Tungstic aeid/5.4 (fioglihmllzo 155 Air/30." 260 Air/30..- 1,426 N7/882.7 1.0 15.3 1,405 1,431 0.98

y o e 6 Thorium nltrate/ 150 Air/15 230 Air/15... 1,426 N1/8 76.1 1.121.3 2,344 1,891 1.24

10. 7 Tantlalurrli (Oxalic acid/5) NHa/IL. 242 Air/40... 1,426 Nz/8 85.30.9 13.6 1,362 1,503 0.91

oxa ate 4. 8 Ammonium (Ammonium 165 Air/20.-. 238 Air/11L 1,426 Nz/879.7 0.9 15.0 2,270 1,813 1.25

molybdate/fi.5. hydroxide/7).

In addition to ammonium chloride. "Not determined.

procedure. The impregnating solutions contained 20-25 parts of ammoniumchloride and the number of parts of heavy metal containing compounds asindicated. Dwell time in the drying section was one minute, in the firstheating stage 2 minutes, and in the second heating stage, 30-40 seconds,in each instance.

These yarns can be used to make fiber-reinforced plastic sheets, as bylaying up the fibers with epoxy resin prepolymer and curing agent,followed by curing, as known to the art. The sheets thus produced can beused for x-ray shielding.

EXAMPLE 2 A similar procedure is used for treating rayon cloth EXAMPLE 3Ammonium chloride 19.6 parts Tungstic acid 5.4 parts Ammonium Hydroxide19,6 purt (concentrated) Wetting agent 2.1 parts Water 53.3 ports toproduce carbonized cloth containing heavy metals. 'hp f y f is passedthroughca 'y Samples of rayon (square weave approximately OZ- Whlch 1SSupplled Wlth heated all at C., flowing at /yd cloth about 12 incheswide and 12 inches long are a fate the temperature of the drymg tubeused. The ovens provided have cross-section adequate belhg mamtamed atabout 155 After substahhahyi to receive h l h i h f ldi all of the waterhas been removed from the yarn, which To aid in increasing the take-upof heavy metal salt, i f p f y a dwell time of about 01:16 h theimpregnation is optionally conducted in two steps, 15 f y i and through01/511 whlch P the cloth being dried between the steps. Excess solutionVlded Wlth a nitrogen m p flqwlhg at the fate is removed running thecloth through a hand Of about and WhlCh IS maintained at 1,426 wringenThe yarn is exposed to the oven temperature for 40 sec- When heavyclosely-woven fabrics are used, the heat ohds- Immediately after g gfrom the 01/611, the treatments are preferably conducted in such a wayas yarn is exposed to ambient temperature and atmoto avoid excessivelyrapid temperature rise in the interphereior of the fabric, such as maybe caused by the exother- The yarn thus produced contains 16.0 percenttungmic nature of the chemical transformation and the relasten, 82.5percent carbon and about 1 percent nitrotively poor thermal conductivityof the fabric. This can gen. About 0.4 percent hydrogen is also present.The be done by heating in successive stages at increasingly material iscompletely amorphous and is found to have high temperatures. a breakingstrength of 1,545 grams, denier 1,588 and The fabric thus produced canbe used, 6g 88 tape tenacity 0.97. The conductivity of this material isabout backings, or to wrap cables, or in the form of multiple 152ohms/ft. The yarn is very heat-resistant, and layers sewn together, asx-ray shielding material. The strongly absorbs x-radiation. fabric canalso be used to form reinforced plastic sheet laminates. EXAMPLE 4 Table11 shows the results obtained. Two-ply rayon yarn (denier 1,650/end, 720fila- TABLE 11" First Second Analysis Drying Section Heating StageHeating Stage impregnating Solution Time/Temp Time/Temp Time/Temp Atm/Atml Atml me- Run (aqueous) min C Flow min C Flow sec "C Flow C N talCFH can CFH 9 1st solution:

ammonium sulfate, 10% 8/120 air/50 10/260 air/50 /1345 N,/8 l0 boricacid, 4% cyanoguanidine, 2% zirconium acetate. 10% glacial acetic acid,6% 2nd solution: ammonium sulfate, 10% boric acid, 4% cyanoguanidine, 2%ammonium phosphate, 4% 10 1st solution: 8/120 air/ 10/260 air/50 40/1345N2/8 62.2 4.4 19 thorium nitrate, 10% (2.8 0.4

B, P) boric acid, 4% cyanoguanidine, 2% 2nd solution: ammonium sulfate,10% boric acid, 4% cyanoguanidine, 2% ammonium phosphate, 4% 11 tungsticacid, 5% 10/110 air/50 10/232 air/50 40/1350 N,/8 73.5 0.9 22.8

ammonium chloride. 25%

' not determined ments/ply) is thoroughly impregnated by passing itthrough a dip tank containing a solution having the followingcomposition:

Ammonium chloride l9.6 parts Tungstic acid 4 parts Ammonium hydroxide(35%) 19.6 parts Wetting agent 2 1 parts Water 53.3 pans Conveniently,the yarn is handled in long lengths which are stored on rolls.

The impregnated yarn is passed through a drying oven as described inExample 1, which is supplied with heated air at 120 C. flowing at a rateof 40 CFl-l, the temperature of the drying tube being'maintained atabout 145 C. After substantially all of the water has been removed fromthe yarn, which is accomplished by a dwell time of about one minute, itis conveyed into and through an oven which is supplied with heated airat about 215 C., which is flowing at the rate of about 6 CFl-l. Thetemperature in this oven is maintained at about 265 C., and the yarn isexposed to the oven temperature for about 2 minutes. The yarn,transformed by the heating to a black, lustrous condition, emerges fromthe oven into ambient air.

The yarn thus produced contains 8.2 percent tungsten, 54.1 percentcarbon, 4.4 percent nitrogen and 3.5 percent of hydrogen. The materialis completely amorphous and is found to have a breaking strength of2,180 grams, denier 2,599 and tenacity 0.84. The material iselectrically non-conductive and quite heat-resistant.

I claim:

1. A process of thermochemically converting regenerated cellulose fiberstarting material to corresponding black, conductive, heavymetal-containing fiber material, which consists essentially inimpregnating clean starting material with a solution of acetate,nitrate, oxalate, oxide, fluoride or chloride salt of heavy metal whichis capable of reacting with carbon to form a refractory high-temperaturestable carbide, and water-soluble ammonium, methylamine or ethylaminesalt with strong acid that is capable of rendering the fibersnonflammable, drying and heating the dry saltimpregnated fiber materialfor about 1 to minutes at an effective temperature of about 200 to 350C. and in the presence of air, the conditions being controlled so as tocause the fiber material to pass through a pyrolytic stage, whileavoiding destructive exotherm, to result in a flexible, black,electrically non-conducting fiber material having a fiber carbon contentin the range of about 50 to 65 percent and a nitrogen content of atleast about 1 percent, both exclusive of heavy metal; and subsequentlycarbonizing this fiber material by rapidly heating for about 5 secondsto 10 minutes in an essentially non-oxidizing environment to a finaltemperature in the range of l,200 to 1,500 but below the temperature atwhich reaction of the said heavy metal with carbon occurs to form theheavy metal carbide, to produce electrically conductive, x-rayabsorbent,heat-stable, nitrogen-containing fibers having a fiber carbon content ofabout 40 to 95 percent and a nitrogen content of about 6 to 6 percent,and containing from 1 to 50 percent by weight of heavy metal chemicallybound to said fiber in other than carbide form.

2. A process according to claim 1, in which the heavy metal is afissionable actinide.

3. A process according to claim 1, in which the fiber is firstimpregnated with a heavy metal salt and then impregnated with a salt ofa strong acid and a nitrogenous base.

4. A process according to claim '1, in which the fiber is firstimpregnated with a salt of a strong acid and a nitrogenous base and thenis impregnated with a heavy metal salt.-

5. A process according to claim 1, in which the fiber is simultaneouslyimpregnated with a heavy metal salt and a salt of a strong acid and anitrogenous base.

6. A process according to claim 5, in which the salt of a strong acidand a nitrogenous base is ammonium chloride.

7. A process according to claim 1, in which both heating steps are lessthan 2 minutes in duration.

8. A process according to claim 1, in which a mixture of two or moreheavy metal salts is used.

9. A process according to claim 1, in which the water-soluble salt of astrong acid and a nitrogenous base is impregnated in the fiber to theextent of 5 to 45 percent by weight.

10. In the process according to claim 1 for thermochemically convertingregenerated cellulose fiber starting material to corresponding heavymetal-containing fibrous material without loss of fiber identity, thestep which consists essentially in heating dry saltimpregnated fiber forabout 1 to 10 minutes at an effective temperature of at least about 200C. up to 350 C. and in the presence of air, the conditions being socontrolled as to cause the fiber material to pass through a pyrolyticstage while avoiding destructive exotherm to result in a flexible,black, electrically non-conducting, x-ray-absorbent fiber materialhaving a fiber carbon content in the range of about 50 to 65 percent anda nitrogen content of at least about 1 percent, the remainder of thematerial being essentially only oxygen and hydrogen exclusive of heavymetal; and containing substantially all of the heavy metal with which itwas originally impregnated, in amount of about 1 to 50 percent byweight.

11. A process according to claim 10, in which the fiber is firstimpregnated with the heavy metal salt and then with the salt of a strongacid and a nitrogenous base.

12. A process according to claim 10, in which the fiber issimultaneously impregnated with the heavy metal salt and the salt of astrong acid and a nitrogenous base. g

13. A process according to claim 10, in which the salt of a strong acidand a nitrogenous base is ammonium chloride.

14. A process according to claim 10, in which a mixture of two or moreheavy metal salts is used.

15. A process according to claim 10, in which the heavy metal isfissionable actinide.

16. A process according to claim 1, in which the heavy metal istungsten.

17. A process according to claim 1, in which the heavy metal is uranium.

18. A process according to claim 1, in which the heavy metal istantalum.

19. A process according to claim 1, in which the heavy metal is thorium.

20. A process according to claim 1, in which the heavy metal iszirconium.

21. A process according to claim 1, in which the heavy metal ismolybdenum.

22. A process of thermochemically converting regenerated cellulose fiberstarting material to corresponding black, conductive, heavymetal-containing fibrous material which consists essentially inimpregnating clean starting material with acetate, nitrate, oxalate,oxide, fluoride or chloride salt of heavy metal which is capable ofreacting with carbon to form a refractory high-temperature stablecarbide, and water-soluble ammonium. methylamine or ethylamine salt withstrong acid that is capable of rendering the fibers nonflamma- 12 ble,drying and carbonizing the dry salt-impregnated fiber material byrapidly heating for about 5 seconds to 10 minutes in a non-oxidizingenvironment to a final temperature in the range of 1,200 to l,500 C.,but below the temperature at which reaction of the said heavy metal withcarbon occurs to form the heavy metal carbide, to produce electricallyconductive, xray-absorbent, heat-stable nitrogen-containing fibershaving a carbon content of about 40 to percent and nitrogen content ofabout 1% to 6 percent and containing from 1 to 50 percent by weight ofheavy metal chemically bound to said fibers in other than carbide form.

2. A process according to claim 1, in which the heavy metal is afissionable actinide.
 3. A process according to claim 1, in which thefiber is first impregnated with a heavy metal salt and then impregnatedwith a salt of a strong acid and a nitrogenous base.
 4. A processaccording to claim 1, in which the fiber is first impregnated with asalt of a strong acid and a nitrogenous base and then is impregnatedwith a heavy metal salt.
 5. A process according to claim 1, in which thefiber is simultaneously impregnated with a heavy metal salt and a saltof a strong acid and a nitrogenous base.
 6. A process according to claim5, in which the salt of a strong acid and a nitrogenous base is ammoniumchloride.
 7. A process according to claim 1, in which both heating stepsare less than 2 minutes in duration.
 8. A process according to claim 1,in which a mixture of two or more heavy metal salts is used.
 9. Aprocess according to claim 1, in which the water-soluble salt of astrong acid and a nitrogenous base is impregnated in the fiber to theextent of 5 to 45 percent by weight.
 10. In the process according toclaim 1 for thermochemically converting regenerated cellulose fiberstarting material to corresponding heavy metal-containing fibrousmaterial without loss of fiber identity, the step which consistsessentially in heating dry salt-impregnated fiber for about 1 to 10minutes at an effective temperature of at least about 200* C. up to 350*C. and in the presence of air, the conditions being so controlled as tocause the fiber material to pass through a pyrolytic stage whileavoiding destructive exotherm to result in a flexible, black,electrically non-conducting, x-ray-absorbent fiber material having afiber carbon content in the range of about 50 to 65 percent and anitrogen content of at least about 1 percent, the remainder of thematerial being essentially only oxygen and hydrogen exclusive of heavymetal; and containing substantially all of the heavy metal with which itwas originally impregnated, in amount of about 1 to 50 percent byweight.
 11. A process according to claim 10, in which the fiber is firstimpregnated with the heavy metal salt and then with the salt of a strongacid and a nitrogenous base.
 12. A process according to claim 10, inwhich the fiber is simultaneously impregnated with the heavy metal saltand the salt of a strong acid and a nitrogenous base.
 13. A processaccording to claim 10, in which the salt of a strong acid and anitrogenous base is ammonium chloride.
 14. A process according to claim10, in which a mixture of two or more heavy metal salts is used.
 15. Aprocess according to claim 10, in which tHe heavy metal is fissionableactinide.
 16. A process according to claim 1, in which the heavy metalis tungsten.
 17. A process according to claim 1, in which the heavymetal is uranium.
 18. A process according to claim 1, in which the heavymetal is tantalum.
 19. A process according to claim 1, in which theheavy metal is thorium.
 20. A process according to claim 1, in which theheavy metal is zirconium.
 21. A process according to claim 1, in whichthe heavy metal is molybdenum.
 22. A process of thermochemicallyconverting regenerated cellulose fiber starting material tocorresponding black, conductive, heavy metal-containing fibrous materialwhich consists essentially in impregnating clean starting material withacetate, nitrate, oxalate, oxide, fluoride or chloride salt of heavymetal which is capable of reacting with carbon to form a refractoryhigh-temperature stable carbide, and water-soluble ammonium, methylamineor ethylamine salt with strong acid that is capable of rendering thefibers nonflammable, drying and carbonizing the dry salt-impregnatedfiber material by rapidly heating for about 5 seconds to 10 minutes in anon-oxidizing environment to a final temperature in the range of 1,200*to 1, 500* C., but below the temperature at which reaction of the saidheavy metal with carbon occurs to form the heavy metal carbide, toproduce electrically conductive, x-ray-absorbent, heat-stablenitrogen-containing fibers having a carbon content of about 40 to 95percent and nitrogen content of about 1/2 to 6 percent and containingfrom 1 to 50 percent by weight of heavy metal chemically bound to saidfibers in other than carbide form.